Lithium-ion electrolytes with fluoroester co-solvents

ABSTRACT

An embodiment lithium-ion battery comprising a lithium-ion electrolyte of ethylene carbonate; ethyl methyl carbonate; and at least one solvent selected from the group consisting of trifluoroethyl butyrate, ethyl trifluoroacetate, trifluoroethyl acetate, methyl pentafluoropropionate, and 2,2,2-trifluoroethyl propionate. Other embodiments are described and claimed.

PRIORITY CLAIM

This application claims the benefit of U.S. Provisional Application No. 60/897,316, filed 25 Jan. 2007.

GOVERNMENT INTEREST

The invention claimed herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.

FIELD

The present invention relates to lithium-ion electrolytes, with application to lithium-ion batteries.

BACKGROUND

Future NASA missions to explore Mars, the Moon, and the outer planets, are expected to require rechargeable batteries that can operate over a wide temperature range (−60° C. to +60° C.) to satisfy the requirements of various applications, including landers, rovers, and penetrators. Some future applications typically may require high specific energy batteries that can operate at very low temperatures, while still providing adequate performance and stability at higher temperatures. In addition, many of these applications envisioned by the Exploration Systems Research and Technology (ESRT) program of NASA will require improved safety, due to their use by humans. The performance of currently known state-of-art lithium-ion systems is severely limited at temperatures below −40° C. and above +30° C. The limited performance at very low temperatures is believed to be due to poor electrolyte conductivity, poor lithium intercalation kinetics over the electrode surface layers, and poor ionic diffusion in the electrode bulk.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a battery according to an embodiment.

FIGS. 2-10 illustrate discharge curves of embodiments.

DESCRIPTION OF EMBODIMENTS

In the description that follows, the scope of the term “some embodiments” is not to be so limited as to mean more than one embodiment, but rather, the scope may include one embodiment, more than one embodiment, or perhaps all embodiments.

FIG. 1 illustrates in a simplified pictorial fashion a portion of a lithium-ion battery according to an embodiment, comprising anode 102, cathode 104, and separator 106 with an electrolyte comprising lithium ions 108 that move through separator 106 from anode 102 to cathode 104 during a battery discharge, and from cathode 104 to anode 102 during a battery charge. For some batteries with a cylindrical form factor, another separator layer may be adjacent to cathode 104, and all four layers may be wound into a cylindrical form. However, embodiments are not limited to any particular form factor.

For some embodiments, anode 102 comprises MCMB (Mesocarbon Microbeads) carbon, and cathode 104 comprises LiNi_(0.8)CO_(0.2)O₂ (lithium nickel cobalt oxide). Other embodiments may use other kinds of anodes or cathodes. For example, other embodiments may use cathodes comprising lithium cobalt oxide (LiCoO₂), lithium manganese oxide (LiMn₂O₄), lithium nickel cobalt aluminum oxide (LiNiCoAlO₂), lithium iron phosphate (LiPO₄), and lithium nickel cobalt manganese oxide (LiNiCoMnO₂), and LiFePO₄ to name a few examples. As another example, the stoichiometries for nickel, cobalt, and manganese may vary, so that for some embodiments a cathode may comprise LiMn_(x)O₄ and LiNi_(y)Co_(z)Mn_(u)O₂, where x, y, z, and u denote variable stoichiometries.

The lithium-ion electrolyte solution comprises a lithium salt, such as lithium hexafluoro phosphate (LiPF₆), dissolved into a co-solvent to provide a lithium-ion electrolytic solution (electrolyte). Other embodiments may use other types of lithium salt, such as lithium tetrafluoroborate (LiBF₄), lithium bis(oxalate) borate (LiBOB, or LiB(C₂O₄)₂), lithium hexafluoroarensate (LiAsF₆), lithium perclorate (LiClO₄), lithium trifluoromethanesulfonate (LiCF₃SO₃), and lithium bistrifluoromethanesulfonate sulfonyl imide (LiN(SO₂CF₃)₂), to name a few. Also, some embodiments may utilize a mixture of such lithium salts.

During discharge, the reaction CLi_(x)→C+Li⁺+xe⁻, where e⁻ denotes an electron, takes place at anode 102, and the reaction xLi⁺+xe⁻+Li_(1-x)Ni_(0.8)Cu_(0.2)O₂LiNi_(0.8)CO_(0.2)O₂ takes place at cathode 104 (for an embodiment with a cathode comprising LiNi_(0.8)Cu_(0.2)O₂). During charge, these reactions are reversed.

A number of ternary co-solvent embodiments were investigated comprising ethylene carbonate (EC), ethyl methyl carbonate (EMC), and a fluoroester (also referred to as fluorinated ester) solvent selected from the group consisting of 2,2,2-trifluoroethyl butyrate (C₆H₉F₃O₂ or CH₃CH₂CH₂CO₂CH₂CF₃), Chemical Abstract Services registry number CAS # 371-27-7, abbreviated in these letters patent as TFEB; ethyl trifluoroacetate (C₄H₅F₃O₂, or CF₃CO₂CH₂CH₃), CAS #383-63-1, abbreviated in these letters patent as ETFA; 2,2,2-trifluoroethyl acetate (C₄H₅F₃O₂, or CH₃CO₂CH₂CF₃), CAS # 406-95-1, abbreviated in these letters patent as TFEA; and methyl pentafluoropropionate (C₄H₃F₅O₂, or CF₃CF₂CO₂CH₃), CAS# 378-75-6, abbreviated in these letters patent as MPFP. In particular, these co-solvents were blended in carbonate-based electrolytes to give the following electrolyte solutions:

1.0 M LiPF₆ in EC+EMC+TFEB (20:60:20 v/v %);

1.0 M LiPF₆ in EC+EMC+TFEB (20:40:40 v/v %);

1.0 M LiPF₆ in EC+EMC+TFEB (20:20:60 v/v %);

1.0 M LiPF₆ in EC+EMC+ETFA (20:60:20 v/v %);

1.0 M LiPF₆ in EC+EMC+TFEA (20:60:20 v/v %);

1.0 M LiPF₆ in EC+EMC+TFEA (20:40:40 v/v %); and

1.0 M LiPF₆ in EC+EMC+MPFP (20:60:20 v/v %).

A number of experimental glass wound (cylindrical) lithium-ion cells, with MCMB carbon anodes and LiNi_(0.8)CO_(0.2)O₂ cathodes, with Li metal reference electrodes, were fabricated to study the above listed electrolytes. Results of those experiments are provided in Table 1. The last two entries in Table 1 are electrolytes comprising 1.0 M LiPF₆ in EC+EMC (20:80 v/v %); and 1.0 M LiPF₆ in EC+DEC (diethyl carbonate)+DMC (dimethyl carbonate) (1:1:1 v/v %). These two electrolytes serve as baselines for comparison.

TABLE 1 Charge-discharge characteristics of experimental lithium-ion cells containing various electrolytes subjected to formation cycling. Charge Discharge Reversible Cummulative Capacity Capacity Irreverisible Couloumbic Charge Capacity Irreverisible Couloumbic Electrolyte (Ah) (Ah) Capacity Efficiency Capacity (Ah) (Ah) Capacity Efficiency Type 1st Cycle 1st Cycle (1st Cycle) (1st Cyle) 5th Cycle 5th Cycle (1st-5th Cycle) (5th Cycle) 1.0 M LiPF6 0.4216 0.3334 0.088 79.08 0.3341 0.3266 0.1241 97.76 EC + EMC (20:80 v/v %) 1.0 M LiPF6 0.47882 0.41059 0.068 85.75 0.41169 0.39789 0.1313 96.65 EC + DEC + DMC (1:1:1 v/v %) 1.0 M LiPF6 0.39576 0.29202 0.104 73.79 0.31029 0.30345 0.1430 97.80 EC + EMC + TFEA (20:60:20 v/v %) 1.0 M LiPF6 0.42394 0.30787 0.116 72.62 0.32546 0.31174 0.2790 95.78 EC + EMC + TFEA (20:40:40 v/v %) 1.0 M LiPF6 0.51793 0.44202 0.076 85.34 0.43826 0.43123 0.1140 98.40 EC + EMC + TFEB (20:60:20 v/v %) 1.0 M LiPF6 0.53078 0.44252 0.088 83.37 0.44440 0.43290 0.1465 97.41 EC + EMC + TFEB (20:40:40 v/v %) 1.0 M LiPF6 0.42751 0.30552 0.122 71.47 0.29636 0.28616 0.1875 96.56 EC + EMC + TFEB (20:20:60 v/v %) 1.0 M LiPF6 0.48459 0.35629 0.128 73.53 0.34877 0.33726 0.1894 96.70 EC + EMC + ETFA (20:60:20 v/v %) 1.0 M LiPF6 0.48023 0.29415 0.186 61.25 0.21281 0.18844 0.3255 88.55 EC + EMC + MPFP (20:60:20 v/v %)

As shown in Table 1, all cells displayed good reversibility at room temperature and minimal reactivity during the formation cycling, with the exception of the cell containing the MPFP-based electrolyte. The high coulombic efficiency and comparable irreversible capacity losses are indirectly related to the overall stability of the solutions and the electrode filming characteristics. Note that the fluoroester-based solutions do not display deleterious effects that may limit the reversible capacity or may hinder the kinetics of the lithium intercalation and de-intercalation processes. In some cases, some cells displayed much higher efficiencies and lower cumulative irreversible capacity losses, such as the cells containing TFEB in low proportion, compared with the comparable baseline solution.

Of the electrolytes listed above, the embodiment formulations comprising 1.0 M LiPF₆ in EC+EMC+TFEB (20:60:20 v/v %) and 1.0 M LiPF₆ in EC+EMC+TFEA (20:60:20 v/v %) yielded the greatest improvements in terms of performance, resulting in improved low temperature performance and greater high temperature resilience. As shown in FIG. 2, good reversibility was observed with the cell containing the 1.0 M LiPF₆ EC+EMC+TFEB (20:60:20 v/v %) electrolyte, being comparable with the baseline 1.0 M LiPF₆ electrolyte formulation. In FIG. 2, curve 201 shows voltage vs. discharge capacity (in Ampere-hour units) for the first discharge cycle, and curve 202 shows this relationship for the fifth discharge cycle. A 25 mA current was used to charge the cell to 4.10 V, and discharge was at 25 mA to 2.75 V, all at a room temperature of 23° C.

FIG. 3 provides discharge curves for the above listed electrolytes at a temperature of −20° C., for a 25 mA discharge current to 2.00 V. The key to the curves is shown as an inset in FIG. 3. FIG. 3 shows good low temperature performance with the cells containing the ethyl trifluoroacetate (ETFA) and trifluoroethyl butyrate (TFEB) co-solvents when used in 20 vol % compared with the ternary carbonate baseline, with over 85% of the room temperature capacity being delivered at −20° C. using moderate rates (a discharge rate of approximately C/16). The performance enhancement of these electrolytes is more dramatic when compared to the binary baseline formulation of comparable EC-content, especially with respect to the degree of polarization observed with the TFEB-containing solution.

The cells were evaluated at the lower temperature of −40° C., as shown in FIG. 4. Excellent performance was again obtained with the cells containing the ethyl trifluoroacetate (ETFA) and trifluoroethyl butyrate (TFEB) co-solvents with over 70% of the room temperature capacity being delivered at a discharge rate of approximately C/16. Again, for embodiments with the TFEB-containing electrolyte, much less polarization was observed compared to the baseline, with a higher operating voltage throughout the discharge.

FIG. 5 provides discharge curves for the cells evaluated at −40° C., but using higher discharge rates (corresponding to a discharge rate of about C/8). The cell containing the TFEB in 20 vol % proportion dramatically outperformed all other cells, displaying over a six-fold increase in capacity when compared to the ternary baseline electrolyte. This dramatic improvement, in terms of overall capacity delivered and the extent of polarization observed, was also observed relative to the binary baseline composition with comparable EC-content, clearly demonstrating a beneficial effect of the fluoroester co-solvent upon the low temperature performance. (So as not to obscure the axes and other curves, in FIG. 5 only a few data points are shown for curve 9.)

FIG. 6 provides discharge curves evaluated at −60° C. using lower discharge rates (corresponding to a discharge rate of about C/30). The cell containing the TFEB in 20 vol % proportion again dramatically outperformed all other cells, displaying over a four-fold increase in capacity when compared to the binary baseline electrolyte. In contrast, the ternary baseline electrolyte delivered negligible capacity, since the higher EC-content present in this formulation results in lower ionic conductivity due to the higher viscosity at these temperatures.

As seen from FIGS. 2-6, embodiments with ethyl trifluoroacetate (ETFA) and trifluoroethyl butyrate (TFEB) co-solvents have been demonstrated to result in enhanced performance at low temperature compared with all-carbonate-based electrolyte baseline formulations. These results are summarized in Tables 2A, 2B, and 2C. It is believed that these results may presumably be due to the favorable physical properties of the fluoroester co-solvents coupled with desirable film-forming characteristics at the electrode-electrolyte interface, resulting in improved lithium kinetics. This has been indirectly confirmed by electrochemical characterization of the cells, including electrochemical impedance spectroscopy, Tafel polarization measurements, and linear polarization measurements.

TABLE 2A Low Temperature Discharge Performance of MCMB-LiNiCoO₂ cells comprising fluoroester-based electrolytes. 1.0M LiPF6 1.0M LiPF6 1.0M LiPF6 1.0M LiPF6 EC + EMC + TFEB EC + EMC + TFEB EC + EMC + TFEB EC + EMC + TFEA (20:60:20 v/v %) (20:40:40 v/v %) (20:20:60 v/v %) (20:60:20 v/v %) Current Capacity Capacity Capacity Capacity TEMP (mA) (Ahr) % (Ahr) % (Ahr) % (Ahr) %   23 C. 25 0.4312 100 0.4329 100 0.3035 100 0.3117 100 −20 C. 25 0.3718 86.23 0.2727 63.00 0.2519 83.02 0.2544 81.59 −20 C. 50 0.3528 81.82 0.2314 53.45 −40 C. 25 0.3166 73.41 0.1064 24.57 0.1756 57.87 0.1657 53.17 −40 C. 50 0.2631 61.01 0.0364 8.40 0.0777 25.61 0.0518 16.61 −50 C. 25 0.2024 467.93 0.0347 8.02 0.0288 9.48 0.0208 6.66 −50 C. 50 0.0324 7.52 0.0155 3.58 −60 C. 5 0.2723 63.14 0.1084 25.03 0.0633 20.88 0.0456 14.64 −60 C. 10 0.1918 44.47 0.0210 4.85 0.0091 2.99 0.0055 1.75 −60 C. 25 0.0209 4.85 0.0080 1.84 0.0006 0.21 0.0002 0.08

TABLE 2B Low Temperature Discharge Performance of MCMB-LiNiCoO₂ cells comprising fluoroester-based electrolytes. 1.0M LiPF6 1.0M LiPF6 1.0M LiPF6 EC + EMC + TFEA EC + EMC + ETFA EC + EMC + MPFP (20:40:40 v/v %) (20:60:20 v/v %) (20:60:20 v/v %) Capacity Capacity Capacity TEMP Current (mA) (Ahr) % (Ahr) % (Ahr) %   23 C. 25 0.3117 100 0.3373 100 0.1884 100 −20 C. 25 0.2544 81.59 0.3051 90.48 0.1158 61.44 −20 C. 50 0.2764 81.97 0.0681 36.11 −40 C. 25 0.1657 53.17 0.2580 76.49 0.0038 2.02 −40 C. 50 0.0518 16.61 0.1983 58.79 0.0001 0.03 −50 C. 25 0.0208 6.66 0.1221 28.31 0.0023 1.22 −50 C. 50 0.0088 2.03 0.0000 0.01 −60 C. 5 0.04567 14.64 0.1749 40.55 0.0000 0.02 −60 C. 10 0.0055 1.75 0.0822 19.07 0.0000 0.00 −60 C. 25 0.0002 0.08 0.0018 0.41 0.0000 0.01

TABLE 2C Low Temperature Discharge Performance of MCMB-LiNiCoO₂ cells comprising fluoroester-based electrolytes. 1.0M LiPF6 1.0M LiPF6 EC + EMC EC + DEC + DMC (20:80 v/v %) (1:1:1 v/v %) Current Capacity Capacity TEMP (mA) (Ahr) % (Ahr) %   23 C. 25 0.3266 100 0.3979 100 −20 C. 25 0.2751 84.21 0.3399 85.43 −20 C. 50 0.3272 82.24 −40 C. 25 0.2023 61.93 0.2659 66.82 −40 C. 50 0.1217 37.27 0.0282 7.09 −50 C. 25 0.0736 22.54 0.0102 2.56 −50 C. 50 0.0001 0.02 −60 C. 5 0.1345 41.17 0.0063 1.58 −60 C. 10 0.0315 9.66 0.0000 0.01 −60 C. 25 0.0021 0.65 0.0000 0.00

In addition to evaluating the low temperature performance of the cells, the high temperature resilience of the cells was tested by subjecting them to increasingly higher temperature storage periods. Each storage period consisted of storing the cells in a full state of charge for 10 days in an open circuit condition. Although it would be expected that the performance degradation is less pronounced if the cells are stored in a partial state of charge, a fully charged condition was selected for the test to accelerate any deleterious electrochemical electrolyte reactions. As illustrated in Tables 3A, 3B, and 3C, the cell containing the 1.0 M LiPF₆ in EC+EMC+TFEB (20:60:20 v/v %) electrolyte sustained the least amount of performance degradation as a result of the high temperature storage testing. For example, after being subjected to 10 days at 55° C., the TFEB-containing cells retained about 75% of the reversible capacity exhibited prior to the storage period, whereas the binary baseline formulation only delivered about 59% of their initial capacity.

TABLE 3A Discharge performance of MCMB-LiNiCoO₂ cells comprising fluoroester- based electrolytes after being subjected to high temperature storage periods (55° C. to 65° C.). 1.0M LiPF6 1.0M LiPF6 1.0M LiPF6 EC + EMC + TFEA EC + EMC + TFEA EC + EMC (20:60:20 v/v %) (20:40:40 v/v %) (20:80 v/v %) % of % of % of Ah initial Ah initial Ah initial Prior to 20 C. 25 mA 0.2810 100.00 0.2803 100.00 0.3032 100.00 Storage Tests 50 mA 0.26904 95.89 0.2633 93.91 0.2879 94.96 100 mA  0.2591 92.23 0.2446 87.25 0.2695 88.90 After 10 Days 20 C. Residual 0.1004 35.74 0.09550 19.61 0.1515 49.97 Storage at 55 C. 25 mA 0.1355 48.22 0.1084 38.68 0.1794 59.17 50 mA 0.1233 43.87 0.0959 Q34.22 0.1713 56.50 100 mA  0.1081 38.46 0.0764 27.25 0.1585 52.29

TABLE 3B Discharge performance of MCMB-LiNiCoO₂ cells comprising fluoroester- based electrolytes after being subjected to high temperature storage periods (55° C. to 65° C.). 1.0M LiPF6 1.0M LiPF6 1.0M LiPF6 EC + EMC + TFEB EC + EMC + TFEB EC + EMCMPFP (20:60:20 v/v %) (20:40:40 v/v %) (20:60:20 v/v %) % of % of % of Ah initial Ah initial Ah initial Prior to 20 C. 25 mA 0.3723 100.00 0.3589 100.00 0.1077 100.00 Storage 50 mA 0.3656 98.22 0.3513 97.87 0.0907 84.20 Tests 100 mA  0.3575 96.04 0.33760 93.89 0.0805 74.71 After 10 20 C. Residual 0.2463 66.16 0.2053 57.19 0.0173 16.05 Days 25 mA 0.2815 75.63 0.2594 72.28 0.0635 58.91 Storage 50 mA 0.2740 73.61 0.2546 70.94 0.0580 53.83 at 55 C. 100 mA  0.2582 69.37 0.2413 67.24 0.0505 46.90 After 10 20 C. Residual 0.1441 38.70 0.1078 30.04 0.0001 0.14 Days 25 mA 0.1724 46.32 0.1492 41.57 0.0350 32.50 Storage 50 mA 0.1566 42.05 0.1326 36.95 0.0298 27.63 at 60 C. 100 mA  0.1297 34.83 0.1018 28.35 0.0141 13.07 After 10 20 C. Residual 0.0776 20.83 0.0140 3.90 0.0014 1.27 Days 25 mA 0.0860 23.09 0.0226 6.30 0.0124 11.47 Storage 50 mA 0.0451 12.10 0.0090 2.52 0.0032 2.93 at 65 C. 100 mA  0.0108 2.91 0.0046 1.29 0.0044 4.12

TABLE 3C Discharge performance of MCMB-LiNiCoO₂ cells comprising fluoroester-based electrolytes after being subjected to high temperature storage periods (55° C. to 65° C.). 1.0M LiPF6 1.0M LiPF6 EC + EMC + ETFA EC + EMC + TFEB (20:60:20 v/v %) (20:20:60 v/v %) % of % of Ah initial Ah initial Prior to 20 C. 25 mA 0.2382 100.00 0.2624 100.00 Storage 50 mA 0.2267 95.15 0.2209 84.17 Tests 100 mA  0.2009 84.32 0.646 24.61 After 10 20 C. Residual 0.0208 8.75 0.1117 42.57 Days 25 mA 0.1130 47.44 0.1505 57.34 Storage 50 mA 0.1004 42.13 0.1210 46.10 at 55 C. 100 mA  0.0803 33.71 0.0680 25.90 After 10 20 C. Residual 0.00 0.00 0.0400 15.23 Days 25 mA 0.0281 11.81 0.0679 25.89 Storage 50 mA 0.0138 5.80 0.0386 14.69 at 60 C. 100 mA  0.0076 3.19 0.0127 4.85 After 10 20 C. Residual 0.00 0.00 0.0038 1.43 Days 25 mA 0.0071 2.96 0.0081 3.08 Storage 50 mA 0.0042 1.76 0.0042 1.62 at 65 C. 100 mA  0.0024 0.99 0.0006 0.24

As described so far, a number embodiments comprising fluoroester-based co-solvents in Li-ion electrolytes, including trifluoroethyl butyrate (TFEB), ethyl trifluoroacetate (ETFA), trifluoroethyl acetate (TFEA), and methyl pentafluoropropionate (MPFP), were found to provide good performance over a wide range of temperatures. It is believed that these embodiments have good safety characteristics, due to their low flammability associated with their halogenated nature. Of the solvents described so far, trifluoroethyl butyrate (TFEB) was demonstrated to yield the best performance, with improved low temperature capability and high temperature resilience. In addition to the above-described embodiments, other embodiments utilizing TFEB as a co-solvent have been found by developing multi-component electrolytes which also contain non-halogenated esters, film forming additives, thermal stabilizing additives, and flame retardant additives.

Several factors may influence the low temperature performance of lithium-ion cells, including: lithium ion mobility in the electrolyte solution (electrolyte conductivity); cell design; electrode thickness; separator porosity; and separator wetting properties. Of these parameters, the electrolyte properties are presumably the most dominant, in that sufficient conductivity is a necessary condition for good performance at low temperatures. In designing electrolytes with high conductivity at low temperatures, the solvents should possess a combination of several critical properties, such as: high dielectric constant; low viscosity; and adequate coordination behavior; as well as appropriate liquid ranges and salt solubilities in the medium.

Some embodiments comprise a quaternary co-solvent that includes TFEB as well as other non-fluorinated ester co-solvents, such as methyl butyrate (MB), ethyl propionate (EP), and propyl butyrate (PB). These non-fluorinated esters may improve the low temperature performance, due to their low viscosity and melting points. Furthermore, some embodiments make use of vinylene carbonate (VC) in conjunction with fluorinated esters, with the intent of producing robust solid electrolyte interface (SEI) layers on the electrodes. VC has previously been reported to be an effective additive in improving the high temperature cycle life and storage characteristics, and is believed to sacrificially polymerize on the electrode surfaces, producing protective films preventing further electrolyte reaction at the interface.

In addition, some embodiments make use of dimethyl acetamide (DMAc) as an electrolyte additive in conjunction with fluorinated ester co-solvents, which has been demonstrated to be a thermal stabilizing agent, acting to reversibly bind with PF₅, preventing LiPF₆ and carbonate solvents from decomposition at high temperature. It is envisioned that the use of VC, DMAc, or both, may improve the life characteristics and high temperature resilience of electrolyte formulations that also contain fluorinated ester co-solvents, which may impart improved operating temperature range and safety.

Embodiments may also include other fluorinated ester co-solvents in addition to the ones previously investigated, such as 2,2,2-trifluoroethyl propionate (C5H7F3 or CH3CH2CO2CH2CF3), referred to as TFEP. These new electrolyte formulations were compared with the previously described solutions, namely 1.0 M LiPF₆ in EC+EMC+TFEB (20:60:20 v/v %) and 1.0 M LiPF₆ in EC+EMC (20:80 v/v %), which were treated as baseline solutions. Accordingly, the following electrolytic solutions were prepared and evaluated:

1.0 M LiPF₆ in EC+EMC+TFEB+MB (20:40:20:20 v/v %);

1.0 M LiPF₆ in EC+EMC+TFEB+EP (20:40:20:20 v/v %);

1.0 M LiPF₆ in EC+EMC+TFEB+PB (20:40:20:20 v/v %);

1.0 M LiPF₆ in EC+EMC+TFEB (20:60:20 v/v %)+1.5% VC;

1.0 M LiPF₆ in EC+EMC+TFEB (20:60:20 v/v %)+1.5% DMAc;

1.0 M LiPF₆ in EC+EMC+TFEB (20:75:5 v/v %);

1.0 M LiPF₆ in EC+EMC+TFEP (20:60:20 v/v %); and

1.0 M LiPF₆ in EC+EMC+TFEP (20:40:40 v/v %).

These were compared with 1.0 M LiPF₆ in EC+EMC+TFEB (20:60:20 v/v %) and 1.0 M LiPF₆ in EC+EMC (20:80 v/v %), which served as baselines.

In addition to the embodiments described above, some embodiments comprising TFEB may also comprise flame retardant additives, such as triphenyl phosphate, triethyl phosphate, tributyl phosphate, and bis-(2,2,2-trifluoroethyl)methyl phosphonate, to reduce the flammability of the electrolytes.

Lithium-ion batteries with MCMB carbon anodes and LiNi_(0.8)CO_(0.2)O₂ cathodes were fabricated with the electrolytes listed above, and measurements related to the cells are provided in Table 4. As shown in Table 4, all cells with the formulations listed above displayed good reversibility at room temperature, and relatively minimal reactivity during the formation cycling. The high coulombic efficiency and comparable irreversible capacity losses are believed to be indirectly related to the overall stability of the solutions and the electrode filming characteristics.

Note that the fluoroester-based solutions do not display deleterious effects that limit the reversible capacity or hinder the kinetics of the lithium intercalation and de-intercalation processes. From the formation data, it may be observed that: the trifluoroethyl butyrate solutions appear to produce more desirable surface films compared with trifluoroethyl propionate, as suggested by the lower cumulative irreversible capacity losses and higher efficiency values; with regard to the quaternary formulations comprising non-fluorinated ester co-solvents, propyl butyrate displayed the most desirable filming attributes, suggesting enhanced stability compared to methyl butyrate and ethyl propionate; vinylene carbonate appears to be an effective film forming agent, leading to stable cells with low irreversible capacity loss and high efficiency; and DMAc appears to have some reactivity during the formation cycling.

TABLE 4 Charge-discharge characteristics of embodiment lithium-ion cells comprising various electrolytes subjected to formation cycling. Charge Discharge Reversible Cummulative Capacity Capacity Irreverisible Couloumbic Charge Capacity Irreverisible Couloumbic (Ah) (Ah) Capacity Efficiency Capacity (Ah) (Ah) Capacity Efficiency Electrolyte Type 1st Cycle 1st Cycle (1st Cycle) (1st Cyle) 5th Cycle 5th Cycle (1st-5th Cycle) (5th Cycle) 1.0 M LiPF₆ 0.4682 0.4044 0.064 86.39 0.4013 0.3914 0.1136 97.53 EC + EMC (20:80 v/v %) 1.0 M LiPF₆ 0.51793 0.44202 0.076 85.34 0.43826 0.43123 0.1140 98.40 EC + EMC + TFEB (20:60:20 v/v %) 1.0 M LiPF₆ 0.52311 0.44761 0.076 85.57 0.44588 0.43613 0.1238 97.81 EC + EMC + TFEB (20:75:5 v/v %) 1.0 M LiPF₆ 0.49027 0.41108 0.079 83.85 0.40770 0.39872 0.1261 97.80 EC + EMC + TFEB + MB (20:40:20:20 v/v %) 1.0 M LiPF₆ 0.48521 0.40316 0.082 83.09 0.40625 0.39724 0.1282 97.78 EC + EMC + TFEB + EP (20:40:20:20 v/v %) 1.0 M LiPF₆ 0.46187 0.39214 0.070 84.90 0.38955 0.38170 0.1128 97.99 EC + EMC + TFEB + PB (20:40:20:20 v/v %) 1.0 M LiPF₆ 0.49797 0.42529 0.073 85.40 0.41434 0.40711 0.1088 98.25 EC + EMC + TFEB (20:60:20 v/v %) + 1.5% VC 1.0 M LiPF₆ 0.51540 0.43330 0.082 84.07 0.43093 0.41485 0.1553 96.27 EC + EMC + TFEB (20:60:20 v/v %) + 1.5% DMAc 1.0 M LiPF6 0.49419 0.41287 0.081 83.54 0.41866 0.40341 0.1518 96.36 EC + EMC + TFEP (20:60:20 v/v %) 1.0 M LiPF6 0.45809 0.37293 0.085 81.41 0.37525 0.35896 0.1604 95.66 EC + EMC + TFEP (20:40:40 v/v %)

FIG. 7 illustrates discharge curves for the above displayed electrolytes. In FIG. 7, curves 3, 4, 6, and 9 are very close to each other, and it may be difficult to distinguish them from each other. (Curve 9 is on top of curve 3, and consequently the numeric label 9 is not shown among the curves.) As shown in FIG. 7, good reversibility was observed with the cells comprising these electrolyte variations, when compared after the formation cycling. It should be noted that some variation in capacity was due to different electrode weights, and not electrolyte type.

FIG. 8 shows discharge curves at −20° C. Good low temperature performance was generally observed, and was somewhat comparable to the baseline all-carbonate electrolyte. The best performance at −20° C. was exhibited by the cell comprising the EC+EMC+TFEB+EP electrolyte, demonstrating that improved low temperature performance may be obtained with fluorinated and non-fluorinated ester blends, and it may be guessed that further optimization of multi-component carbonate and ester blended electrolytes may yield further improvement in low temperature performance.

When the cells described were evaluated at −30° C., as shown in FIG. 9, good performance was again obtained with the cells comprising the fluorinated ester co-solvents. Note that the cells comprising the EC+EMC+TFEB (20:60:20) electrolyte with either VC or DMAc, or the EC+EMC+TFEB+EP (20:60:20) electrolyte, performed comparable to the baseline EC+EMC (20:80) electrolyte, with all cells providing about 79% to 80% of the room temperature capacity at −30° C. Many cells also performed well at −40° C., with about 69% to 74% of the room temperature capacity being delivered at a discharge rate of about C/16, as shown in FIG. 10.

A number of test cells with lithium-ion electrolytes comprising fluorinated ester co-solvents, including trifluoroethyl butyrate (TFEB) and trifluoroethyl propionate (TFEP), were tested, with their performance described in this specification. These electrolytes were found to deliver good performance over a wide temperature range in experimental lithium-ion cells. The general approach taken in the development of these electrolyte formulations was to optimize the type and composition of the co-solvents in ternary and quaternary solutions, focusing upon adequate stability (e.g., EC content for anode passivation, and EMC content for lowering the viscosity and widening the temperature range, while still providing good stability), enhancing the inherent safety characteristics (incorporation of fluorinated esters), and widening the temperature range of operation (the use of both fluorinated and non-fluorinated esters). Furthermore, the use of electrolyte additives, such as VC (SEI promoter) and DMAc (thermal stabilizing additive), are envisioned to provide enhanced high temperature life characteristics. For some embodiments, the concentrations of VC and DMAc may be relatively low, such as for example in the range of 0.10% and 5.0% by weight. It is anticipated that embodiment electrolyte solutions may offer improved performance when incorporated into hermitically sealed, large capacity, prototype cells, especially if all electrolyte components are highly pure.

Various modifications may be made to the described embodiments without departing from the scope of the invention as claimed below. For example, other non-fluorinated ester co-solvents that may be used are: methyl propionate, methyl acetate, ethyl acetate, propyl acetate, ethyl butyrate, butyl butyrate, and propyl propionate. Furthermore, it should be appreciated that experimental results were described for embodiments with specific concentrations of the co-solvents, but embodiments are not limited to these specific concentrations. For example, for some embodiments the EC content may vary from 10% to 50%, the EMC content may vary from 10% to 80%, the fluorinated ester co-solvent content may vary from 5% to 80%, and the non-fluorinated ester co-solvent content may vary from 10% to 80%. Furthermore, for some embodiments the lithium salt concentration (e.g., LiPF₆) may vary from 0.50 M to 1.50 M.

These concentration ranges are merely given to provide examples, and some embodiments may have different ranges of concentrations. For example, the lithium salt concentration may be between 0.80 M and 1.20 M, where very low temperature performance benefits from the lower end of this range, and where more moderate temperature and high power applications may benefit from the higher end of this range. As another example, the concentration of the non-fluorinated esters may be in the range of 10% to 80%.

Throughout the description of the embodiments, various mathematical relationships are used to describe relationships among one or more quantities. These relationships are in practice not satisfied exactly, and should therefore be interpreted as “designed for” relationships. One of ordinary skill in the art may design various working embodiments to satisfy various mathematical relationships, but these relationships can only be met within the tolerances of the technology available to the practitioner.

Accordingly, in the following claims, it is to be understood that claimed mathematical relationships can in practice only be met within the tolerances or precision of the technology available to the practitioner, and that the scope of the claimed subject matter includes those embodiments that substantially satisfy the mathematical relationships so claimed. 

1. A battery comprising: an anode; a cathode; and a lithium-ion electrolyte comprising a solvent and a lithium salt dissolved in the solvent, the solvent comprising: ethylene carbonate; ethyl methyl carbonate; and a fluorinated ester co-solvent.
 2. The battery as set forth in claim 1, the ethylene carbonate having a volume-volume percentage concentration varying from 10 to 50, the ethyl methyl carbonate having a volume-volume percentage concentration varying from 10 to 80, the fluorinated ester co-solvent having a volume-volume percentage concentration varying from 5 to 80, and the lithium salt having a molarity concentration varying from 0.50 M to 1.50 M.
 3. The battery as set forth in claim 1, the fluorinated ester co-solvent comprising one or more co-solvents selected from the group consisting of trifluoroethyl butyrate, ethyl trifluoroacetate, trifluoroethyl acetate, methyl pentafluoropropionate, and 2,2,2-trifluoroethyl propionate.
 4. The battery as set forth in claim 1, the anode comprising one or more compounds selected from the group consisting of mesocarbon microbeads in carbon, and graphite; and the cathode comprising one or more compounds selected from the group consisting of LiNiCoAlO₂, LiPO₄, LiMn_(x)O₄, LiFePO₄, LiNi_(y)Co_(z)Mn_(u)O₂, where x, y, z, and u denote variable stoichiometries.
 5. The battery as set forth in claim 1, the lithium salt comprising one or more salts selected from the group consisting of lithium hexafluoro phosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄), lithium bis(oxalate) borate (LiBOB, or LiB(C₂O₄)₂), lithium hexafluoroarensate (LiAsF₆), lithium perclorate (LiClO₄), lithium trifluoromethanesulfonate (LiCF₃SO₃), and lithium bistrifluoromethanesulfonate sulfonyl imide (LiN(SO₂CF₃)₂).
 6. The battery as set forth in claim 1, the solvent further comprising one or more non-fluorinated ester co-solvents selected from the group consisting of methyl butyrate, ethyl propionate, propyl butyrate, methyl propionate, methyl acetate, ethyl acetate, propyl acetate, ethyl butyrate, butyl butyrate, and propyl propionate.
 7. The battery as set forth in claim 6, the one or more non-fluorinated ester co-solvents having a volume-volume percentage concentration varying from 10 to
 80. 8. The battery as set forth in claim 6, the solvent further comprising one or more electrolyte additives selected from the group consisting of vinylene carbonate and dimethyl acetamide.
 9. The battery as set forth in claim 8, the one or more electrolyte additives in the solvent having a concentration varying from 0.1% to 5.0% by weight.
 10. A lithium-ion electrolyte comprising a solvent and a lithium salt dissolved in the solvent, the solvent comprising: ethylene carbonate; ethyl methyl carbonate; and a fluorinated ester co-solvent.
 11. The lithium-ion electrolyte as set forth in claim 10, the ethylene carbonate having a volume-volume percentage concentration varying from 10 to 50, the ethyl methyl carbonate having a volume-volume percentage concentration varying from 10 to 80, the fluorinated ester co-solvent having a volume-volume percentage concentration varying from 5 to 80, and the lithium salt having a molarity concentration varying from 0.50 M to 1.50 M.
 12. The lithium-ion electrolyte as set forth in claim 10, the fluorinated ester co-solvent comprising one or more co-solvents selected from the group consisting of trifluoroethyl butyrate, ethyl trifluoroacetate, trifluoroethyl acetate, methyl pentafluoropropionate, and 2,2,2-trifluoroethyl propionate.
 13. The lithium-ion electrolyte as set forth in claim 10, the lithium salt comprising one or more salts selected from the group consisting of lithium hexafluoro phosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄), lithium bis(oxalate) borate (LiBOB, or LiB(C₂O₄)₂), lithium hexafluoroarensate (LiAsF₆), lithium perclorate (LiClO₄), lithium trifluoromethanesulfonate (LiCF₃SO₃), and lithium bistrifluoromethanesulfonate sulfonyl imide (LiN(SO₂CF₃)₂).
 14. The lithium-ion electrolyte as set forth in claim 10, the solvent further comprising one or more non-fluorinated ester co-solvents selected from the group consisting of methyl butyrate, ethyl propionate, propyl butyrate, methyl propionate, methyl acetate, ethyl acetate, propyl acetate, ethyl butyrate, butyl butyrate, and propyl propionate.
 15. The lithium-ion electrolyte as set forth in claim 14, the one or more non-fluorinated ester co-solvents having a volume-volume percentage concentration varying from 10 to
 80. 16. The lithium-ion electrolyte as set forth in claim 14, the solvent further comprising one or more electrolyte additives selected from the group consisting of vinylene carbonate and dimethyl acetamide.
 17. The lithium-ion electrolyte as set forth in claim 16, the one or more electrolyte additives in the solvent having a concentration varying from 0.1% to 5.0% by weight. 